A key requirement on Long Term Evolution (LTE) in 3GPP Wireless Communications Systems is frequency flexibility for transmissions between a radio base station and a mobile terminal over a radio link. For this purpose, carrier bandwidths between 1.4 MHz and 20 MHz are supported, as is both Frequency Division Duplex (FDD) and Time Division Duplex (TDD), so that both paired and unpaired frequency spectrum can be used. For FDD, the downlink, i.e. the link from a base station to a mobile terminal, and uplink, i.e. the link from a mobile terminal to a base station, use different frequencies so called “paired frequency spectrum” and can hence transmit simultaneously. For TDD, uplink and downlink use the same frequency “unpaired” frequency spectrum” and can not transmit simultaneously. Uplink and downlink can however share the time in a flexible way, and by allocating different amounts of time, such as the number of subframes of a radio frame, to uplink and downlink, it is possible to adapt to asymmetric traffic and resource needs in uplink and downlink.
The above asymmetry also leads to a significant difference between FDD and TDD. Whereas for FDD, the same number of uplink and downlink subframes is available during a radio frame, for TDD the number of uplink and downlink subframes may be different. In LTE time is structured into radio frames of 10 ms duration, and each radio frame is further divided into 10 subframes of 1 ms each. One of many consequences of this is that in FDD, a mobile terminal can always send feedback in response to a data packet in an uplink subframe subject to a certain fixed processing delay. In other words, every downlink subframe can be associated to a specific later uplink subframe for feedback generation in way that this association is one-to-one, i.e. to each uplink subframe is associated exactly one downlink subframe. For TDD however, since the number of uplink and downlink subframes during a radio frame may be different, it is in general not possible to construct a such one-to-one association. For the typical case with more downlink subframes than uplink subframes, it is rather so that feedback from several downlink subframes requires to be transmitted in each uplink subframe.
In Evolved Universal Terrestrial Radio Access Network (E-UTRAN), a radio frame of 10 ms duration is divided into ten subframes, wherein each subframe is 1 ms long. In case of TDD, a subframe is either assigned to uplink or downlink, i.e., uplink and downlink transmission cannot occur at the same time. Furthermore, each 10 ms radio frame is divided into two half-frames of 5 ms duration where each half-frame consists of five subframes.
The first subframe of a radio frame is always allocated to downlink transmission. The second subframe is split into three special fields, Downlink Pilot Time Slot (DwPTS), Guard Period (GP) and Uplink Pilot Time Slot (UpPTS), with a total duration of 1 ms.
UpPTS is used for uplink transmission of sounding reference signals and, if so configured, reception of a shorter random access preamble. No data or control signalling can be transmitted in UpPTS.
GP is used to create a guard period between periods of downlink and uplink subframes and may be configured to have different lengths in order to avoid interference between uplink and downlink transmissions and is typically chosen based on the supported cell radius.
DwPTS is used for downlink transmission much like any other downlink subframe with the difference that it has shorter duration.
Different allocations of the remaining subframes to uplink and downlink transmission are supported, both allocations with 5 ms periodicity in which the first and second half-frame have identical structure, and allocations with 10 ms periodicity for which the half-frames are organized differently. For certain configurations the entire second half-frame is assigned to downlink transmission. Currently supported configurations use 5 ms and 10 ms periodicity. In case of 5 ms periodicity, the ratio between downlink and uplink may e.g. be 2/3, 3/2, 4/1, etc. In case of 10 ms periodicity, the ratio between downlink and uplink may e.g. be 5/5, 7/3, 8/2, 9/1 etc.
In the downlink of E-UTRAN, OFDM with a subcarrier spacing of 15 kHz is used. In the frequency dimension the subcarriers are grouped into resource blocks, each containing twelve consecutive subcarriers. The number of resource blocks depends on the system bandwidth, and the minimum bandwidth corresponds to six resource blocks. Depending on the configured cyclic prefix length, a 1 ms subframe contains either 12 or 14 OFDM symbols in time. The term resource block is also used to refer to the two-dimensional structure of all OFDM symbols within a half subframe, times a resource block of subcarriers. The special downlink subframe DwPTS has a variable duration, and can assume lengths of 3, 9, 10, 11 or 12 OFDM symbols for the case with normal cyclic prefix, and 3, 8, 9 or 10 symbols for the case with extended cyclic prefix.
In the uplink of E-UTRAN, SC-FDMA, also referred to as DFT-pre-coded OFDM is used. The underlying two-dimensional (time and frequency) numerology is the same in terms of subcarrier spacing, cyclic prefix lengths and number of OFDM symbols. The major difference is that modulated data symbols to be transmitted in certain OFDM symbols are subject to a DFT and the outputs of the DFT are mapped to the subcarriers.
In order to improve performance of transmission in both the downlink and uplink direction, LTE uses Hybrid Automatic Repeat Request (HARQ). We will here discuss the function of this mechanism for downlink transmission. The basic idea of HARQ is that after receiving data in a downlink subframe the terminal attempts to decode it and then reports to the base station whether the decoding was successful (ACK, acknowledgement) or not (NAK, negative acknowledgement). In case of an unsuccessful decoding attempt the base station thus receives a NAK in a later uplink subframe, and can retransmit the erroneously received data.
Downlink transmissions are dynamically scheduled, i.e. in each subframe the base station transmits control information on which terminals are to receive data and upon which resources in the current downlink subframe. Such a control information message to a terminal is referred to as a downlink assignment. A downlink assignment thus contains information to the terminal about in which resources a subsequent data will be sent, and also information necessary for the terminal to decode the subsequent data, such as modulation and coding scheme. By resources is here meant some set of resource blocks. This control signalling is transmitted in the first 1, 2 or 3 OFDM symbols in each subframe. The data sent to a terminal in a single downlink subframe is often referred to a transport block.
A terminal will thus listen to the control channel, and if it detects a downlink assignment addressed to itself, it will try to decode the subsequent data. It will also generate feedback in response to the transmission, in the form of an ACK or a NAK depending on whether the data was decoded correctly or not. Furthermore, from the control channel resources on which the assignment was transmitted by the base station, the terminal can determine the corresponding uplink control channel resource.
For E-UTRAN FDD the terminal will in response to a detected downlink assignment in subframe n, send an ACK/NAK report in uplink subframe n+4. For the case with so-called Multiple In Multiple Out (MIMO) multi-layer transmission, two transport blocks are transmitted in a single downlink subframe, and the terminal will respond with two ACK/NAK reports in the corresponding uplink subframe.
The assignment of resources to the terminals is handled by the scheduler, which takes into account traffic and radio conditions so as to use the resources efficiently while also meeting delay and rate requirements. Scheduling and control signaling may be done on a subframe to subframe basis. Currently there is no dependency between the downlink assignments sent in the different downlink subframes, i.e. each downlink subframe is scheduled independently of others.
As described above, the first step for a terminal to receive data from the base station in a downlink subframe is to detect a downlink assignment in the control field of a downlink subframe. In the case that the base station sends such an assignment but the terminal fails to decode it, the terminal obviously cannot know that is was scheduled and will hence not respond with an ACK/NAK in the uplink. This situation is referred to as a missed downlink assignment. If the absence of an ACK/NAK can be detected by the base station, it can take this into account for subsequent retransmissions. Typically the base station should at least retransmit the missing packet, but it may also adjust some other transmission parameters.
For FDD a terminal can always respond to a downlink data transmission with an ACK/NAK after a fixed delay of 4 subframes, while for TDD there is not a one-to-one relation between uplink and downlink subframes. This was discussed above. Thus the terminal cannot always send an ACK/NAK in response to a downlink assignment in subframe n in uplink subframe n+4, since this subframe may not be allocated to uplink transmission. Hence each downlink subframe may be associated with a certain uplink subframe subject to a minimum processing delay, meaning that ACK/NAKs in response to downlink assignments in subframe n are reported in subframe n+k with k>3. Furthermore, if the number of downlink subframes is larger than the number of uplink subframes, ACK/NAKs in response to assignments in multiple downlink subframes may need to be sent in a single uplink subframe. For a given uplink subframe, the number of associated downlink subframes depends on the configuration of subframes to uplink and downlink, and can be different for different uplink subframes.
Since downlink assignments can be given independently across downlink subframes, a terminal may be assigned downlink transmissions in multiple downlink subframes that are all to be acknowledged in a single uplink subframe. Hence the uplink control signaling needs to support, in some way, feedback of ACK/NAKs from multiple downlink transmissions from a terminal in a given uplink subframe.
One obvious way to approach the above problem is to allow the terminal to transmit multiple individual (for each downlink transmission) ACK/NAK bits in a single uplink subframe. Such protocols have however worse coverage than transmission of a one or two ACK/NAK reports. In addition, the more ACK/NAKs that are allowed to be transmitted from a single terminal, the more control channel resources need to be reserved in the uplink. To improve control signaling coverage and capacity, it is possible to perform some form of compression, or bundling, of ACK/NAKs. This means that all ACK/NAKs that are to be sent in a given uplink subframe are combined into a smaller number of bits, such as a single ACK/NAK report. As an example, the terminal can transmit an ACK only if the transport blocks of all the downlink subframes were received correctly and hence to be acknowledged. In any other case, meaning that at a NAK for at least one downlink subframe is to be transmitted, a combined NAK is sent for all downlink subframes. As described above, to each uplink subframe in TDD a set of downlink subframes can be associated rather than a single subframe as in FDD, for which downlink transmissions are to be given ACK/NAK response in the given uplink subframe. In the context of bundling this set is often referred to as the bundling window.
FIG. 1a and FIG. 1b illustrates two different uplink (UL): downlink (DL) allocations as an example for how bundling windows are used. Uplink subframes contains an upward directed arrow, downlink subframes contains an downward directed arrow, and DwPTS, GP UpPTS subframes comprises both a downward directed arrow and an upward directed arrow in FIGS. 1a and b. In the examples, the number of associated downlink subframes, K, is different for different subframes as well as for different asymmetries. For the 4DL: 1UL configuration in FIG. 1a, the uplink subframe in each half frame is associated to four downlink subframes (K=4). For the 3DL: 2UL configuration in FIG. 1b, the first uplink subframe in each half frame is associated to two downlink subframes (K=2), while the second is associated with a single DL subframe (K=1).
Another advantage of bundling is that it allows reusing the same control channel signaling formats as for FDD, independently of the TDD uplink/downlink asymmetry. The disadvantage is a loss in downlink efficiency. If the base station receives a NAK it cannot know how many and which downlink subframes were received erroneously and which were received correctly. Hence it may need to retransmit all of them.
A problem with ACK/NAK bundling is that a terminal may miss a downlink assignment, which may not be indicated in the bundled response. For instance, assume that the terminal was scheduled in two consecutive downlink subframes. In the first subframe the terminal misses the scheduling downlink assignment and will not be aware that it was scheduled, while in the second subframe it did successfully receive the data. The terminal will, as a result, transmit an ACK, which the base station will assume holds for both subframes, including data in subframe the terminal was not aware of. As a result, data will be lost. The lost data needs to be handled by higher-layer protocols, which typically takes a longer time than HARQ retransmissions and is less efficient. In fact, a terminal will not transmit any ACK/NAK in a given uplink subframe only if it missed every downlink assignment that was sent during the bundling window associated with the uplink subframe.
Thus, a missed downlink assignment will in general result in block errors that need to be corrected by higher-layer protocols, which in turn has a negative impact on performance in terms of throughput and latency. Also, increasing the delay may cause undesirable interactions with TCP based applications.